Apparatus for electrically measuring strain applied in testing strength of materials



July 12, 1949. w. H. HOPPMANN, n, EI'AL. 2,475,514

APPARATUS FOR ELECTRICALLY MEASURING STRAIN APPLIED IN TESTING STRENGTH 0]? MATERIALS Filed Sept. 21, 1945 2 Sheets-Sheet 1 INVENTORfi. a

. 0 M W an Edward C fay/0r Mmam H H ll, ET AL 2,475,614

EASURING STRAIN July 12, 1949. w. H. HOFFMAN APPARATUS FDR ELECTRICALLY M APPLIED IN TESTING STRENGTH OF MATERIALS 2 Sheets-Sheet 2 Filed Sept. 21. 1945 I 2 h r /will I INVENTORfi. Wil/Azm H. Hagonamfl' 4' Edward C. Kay/or ATTOPNEY Patented July 12, 1949 2&75514 APPARATUS FOR URING STR STRENGTH OF William H.

Hoppmann,

Valley Stream,

Application September 21, 1945,

ELECTRICALLY MEAS- AIN APPLIED IN TESTING MATERIALS II, and Edward C. Taylor,

Serial No. 617,902

Claims. (C1. 73-12) (Granted under the This invention pertains to the art or testing strength of materials, particularly the art of testing tensile strength or materials under impact stress. The invention is embodied in apparatus for electrically measuring strain applied to materials being tested, the apparatus being particularly adapted to make delicate and precise electrical measurements under the shock of impact testing.

In copending application by William H. Hoppmann, II, Serial No. 617,900, filed or even date herewith, and directed to an Impact tester, of which application Serial No. 64,066, filed Decemher 8, 1948, is a continuation in part, there is disclosed a machine operable to test materials under impact. The apparatus of the present invention, although it is not limited to this use. is particularly adapted to be embodied in the machine of the referred to copending applications, and is operable to determine the magnitude of strain applied in a given test.

It is an object of paratus for carrying an electric current between a moving body and a stationary body.

It is an additional object to provide apparatus for determining the tensile force acting on a moving body.

It is another object to provide apparatus for making continuous electrical measurements on a moving body and to record the measurements on stationary recording equipment.

It is a further object to provide apparatus for continuously determining the instantaneous tenpositioned between a plurality of moving bodies, and transferring these electrical measurements to stationary electrical recording equipment.

Still further objects and advantages of this invention, as well as its construction, arrangement and operation, will be apparent from the following description and claims in connection with the accompanying drawings. in which Figure l is a schematic front elevation of apparatus embodying the invention,

Figure 2 is a schematic side elevation of the apparatus shown in Figure 1.

Figure 3 is a view in perspective, with certain parts broken away, illustrating a detail oi the invention,

Figure 4 is a cross-sectional plan, the line 6-4 of Figure 1, and

Figure 5 is a wiring diagram, partially schematic, showing an electric circuit suitable for practice of the invention.

The invention is described taken along herein with specific not of March amended April 36, 1928;

this invention to provide ap- 3, 1883, as 3'") 0. G. 75'!) reference to the impact testing equipment disclosed in the hereinbeiore mentioned patent application, Serial No. 617,900. However, the invention can be applied to electrical determinations and electrical measurements on all types of moving bodies: it necessary with suitable minor changes and adaptations in the apparatus described herein.

In Figures 1 and 2 there is shown in outline the impact testing equipment hereinbefore referred to, with the electrical measuring apparatus of the present invention forming 9, part thereof. The testing equipment comprises a tower generally designated l I, which extends upwardly from the anvil I! to a predetermined height, this construction being the same as the apparatus of copending applications Serial No.

No. 64,066 hereinbeiore referred to. The specimen assembly 13, is also similar to the disclosure of the copending applications, and comprises the hammer -or tup l5 and the weight l6, attached to respective opposite ends of a specimen of the material being tested, the specimen being enclosed inside the spacer I1 and therefore not shown. The specimen assembly i3 is dropped from a predetermined height in the tower H, and is guided by the rails ll of the tower ll toward the anvil I! with the weight i6 is advance and the hammer l6 trailing. The weight I6 is contoured to enter the aperture of the anvil I1 without interference, and the hammer i5 is contoured to extend laterally and overhang the sides of aperture 65 whereby it strikes the anvil i2. Tensile stress is applied to the specimen of the assembly H as a result of the impact, by the masses or weight it and hammer l5 moving relatively away from each other.

Under practice of the present invention, attachment of either the hammer l5 or the weight ii to the specimen is through the "weigh bar" It, so called, the weigh bar or member ID being attached between the weight 16 and specimen in the illustrated construction. One end of the specimen is attached to the member I! by being screwed into the threads 28, Figure 3. The other end of member l6, embodying the screw threads 21, is screwed into the weight l6. As a result of the disclosed construction, member ID is subjected to the same tension as is applied to the test specimen.

Between the end attachments 21 and 26, the member l6 embodies the bar 26, with a crosssectlonal area much greater than the specimen. which is therefore great enough to withstand one side thereof thus adapted to scribed more fully hereinafter.

A set of contacts 22 auxiliary to each track 2|, is disposed preferably on the opposite side or the path of the tower H proximate to the anvil i2 in position for electrical connection at the time when the hammer Ii strikes the anvil l2 to make the test. Each set of contacts 22 comprises a pair or opposed strips of conductor material that exmoving relatively away from each other, and physically in Opposite directions due to rebound of the hammer.

The weigh bar, generally de ignated I8, is preferably constructed of steel, a bar 26 with an externally threaded portion 21 comprise each a fine high-resistant wire, are fas- 2l as shown in the Figcement.

An arm ll is provided for each track 20. and is attached to the watch baill adjacent one end extend outwardly at right angles to At one end 01' each arm I! is is el connected to the strain gages 2! by means or wire ll and spring lel with the shoe 2| by means of leads ll, Figure 4.

Shoes 21 are placed in contact with their respective tracks 20 at the sembly it is time a positioned in the tiling instruments are thereby conditioned to the electrical characcontinuously until the test is completed. The tracks 2| accordingly extend downwardly from the top or the tower H thromhout the Contacts 22 extend upwardly from anvil l: a sufllcient distance to be eng il-gspective terminal pins 28 before are accuratelymeasured. The sets oi contacts 22 extend downwardly a suilicient distance to maintain contact with eir respective pins 2! throughout the continued travel oi assembly ll after impact with the anvil I2. Contact between each pin 23 and its caiact 22 continues at least until the specimen omes ruptured and the weight it thereby becomes parted from the ammer l8.

The wiring diagram shown in Figure 5 indicates a means whereby electric impulses generated in the strain gages are transformed into readable and recordable measurements. The strain gages are shown in Figure 5 as four resisttimes 29 in series. A sliding contact between shoes 2i and the tracks 2|), and also contact between pins 23 and contacts 22, is indicated by arrows 35. The circuit in Figure 5 is essentially a potentiometer with the primary circuit passing through battery 35, switch 31, resistance 38, resistance 39, switch 40, strain gages 25 and milliammeter ll. Battery 35 (or alternatively any convenient steady direct current source) is arbitrarily set at a convenient voltage such as, for example, 180 volts. Resistance 38 is a fixed resistance approximately equal to the sum of resistances 29 so that roughly one half the total voltage drop will occur over resistances 29, and resistance 39 is so selected that the voltage across this resistance will be roughly five volts when the primary circuit is completed. An amplifying system is connected to this primary circuit between points 42 and 43, which are so chosen that electrical variations in strain gages 29 and resistance 39 will be amplified and recorded thereby.

The amplifying and recording system comprises essentially an audio-frequency oscillator 44, an impedance coupler 45, a voltage limiting amplifier 46, an attenuator 41, an amplifier and cathode-ray oscillograph 48 and a sweep current generator 49. This amplifier and oscillograph system is conventional; however, this particular amplifying and recording system was selected at least partly because the system as a whole is well adapted to detect and record small electrical variations without being unduly susceptible to large electrical variations. Alternative conventional amplifier systems may be satisfactorily used in this invention but the one outlined in Figure 5 is possibly more advantageous because of this resistance to major electrical fluctuations.

An additional resistance 50, which is extremely large in comparison with resistances 29, e. g., about 500 times as high, is connected in parallel with resistances 29 and the amplifier system hereinbefore described. The purpose of this resistance is to permit calibration in order to determine the change in resistance 29, which is represented by a given displacement in the oscillograph line. Switch 5i permits resistance 50 to be placed in and cut out of the circuit in the calibration operation, and this known variation in the electrical circuit affords a means of determining the sensitivity of the system.

In the use and operation of this apparatus a tensile force acting on the specimen causes a slight variation of the resistance of strain gages 29. This is a common phenomenon and is caused by the stretching of bar 25 within its elastic limit, with its consequent elongation of the wires of gages 29. The variation of the resistance of these gages is relatively small compared to the total resistance of the gage and is practically infinitesimal when compared with the resistance which will be caused if the sliding contacts 35 are not perfectly maintained. Because of this, a faulty contact between track and shoe 2i will result momentarily in an almost infinite resistance which in turn causes an exceptionally powerful signal to be carried to the amplifier system. A finite time is then required for the amplifying system to recover from the comparatively high electrical charge that has been carried to it. Since the impact tester in connection with which this invention is being described contemplates the making of electrical measurements during the very short impact period. a short break in sliding contacts 35 might easily interrupt the oscillograph readings for a suiliciently long period of time to destroy their value. This is particularly true since the impact period is a period of excessive vibrations which tend to increase the possibility of a breaking contact.

Accordingly, there has been provided a by-pass circuit generally designated 52 connected between points 53 and 55 and adapted to carry the current normally accommodated by strain gages 28 during the time when contact 35 is broken. This by-pass circuit is more completely described and disclosed in copending patent application, Serial No. 617,903, entitled By-pass circuit and filed concurrently herewith by Edward C. Taylor and ref erence herein is made to the disclosure contained therein. However, for a more complete understanding of this invention one embodiment of this by-pass circuit is shown in Figure 5 of the present application and will be described hereinafter in outline.

A suitable vacuum tube 55, for example, six 6-N7 tubes in parallel, is connected between points 53 and 54 in series with variable resistance 55. The heating current for the filament is provided, as shown, by a conventional A. C. current source operating in conjunction with a step-down transformer 51. The grid 58 is biased on a D. C. power source, for example, battery 59, which in turn is connected to the primary potentiometer circuit between resistance 39 and switch MI. Power source 59 is so selected that the voltage on grid 58 is roughly 6 volts negative with respect to the voltage on cathode 60. In other words, the power source 59 is so selected that the tube in normal operating conditions is operating a few volts below the cut-oil voltage.

This by-pass circuit operates essentially as follows. When contact 35 is broken, the current through resistance 39 ceases. When this happens, the potential at point Bl between resistance 39 and switch 40 is raised by the amount represented by the voltage drop across resistance 39, namely roughly 5 volts. This in turn is reflected by an increase in the potential of the grid 58 from its normal value of about minus 6 volts with respect to cathode Bil to about minus 1 volt with respect to cathode 50. This is above the cut-off voltage, and accordingly current flows through tube 55 and resistance 56. Tubes 55 and resistance 56 are so selected and adjusted that the current which flows therethrough is substantially the same as the current normally flowing through gages 29. An accurate adjustment of this value may be obtained by adjusting resistance 56 until opening and closing switch 40 causes no effect on the milliammeter II.

In the use and operation of this invention a suitable weigh bar I8 is first prepared as shown and described in connection with Figures 3 and 4. The weigh bar It is attached to the specimen assembly it between the test specimen and the weight l5, and any necessary adjustments of the electrical setup for the test may then be made.

The weigh bar It is placed in the electric circuit as diagramed in Figure 5, and resistance 56 is adjusted so that the by-pass circuit 52 is adapted to carry the some current as the strain gages 29. Next the apparatus is calibrated by maintaining switch II in a closed position and than opening it suddenly. Opening this switch 5| 1 hasthesameeflectontherecordingequipment as increasing resistances II, for example, by applying a strain to the weigh bar ll.

When the equipment has been thus adjusted and calibrated an impact test is made by dropping the specimen assembly II to the anvil II from a predetermined height in the tower I i. The oscillograph curve is preferably photographed to provide a permanent record oi the force-time condi tions in the testing system. The force-time curve thus obtained is a substantially accurate picture of the actual conditions operating on the specimen. There is little or no interference with this curve caused by insecure connection between the weigh bar It and the electric track 2|, thereby indicating that the eilects oi the sliding connection have been largely nullified.

It is to be understood that numerous changes and variations may be made in the mechanical and electrical setup or this invention. For example, this electrical measuring system may be applied to numerous uses other than in connection with an impact tester and when so applied certain variations will be obvious to those skilled in the art. Specifically, in numerous instances it will be possible to omit the by-pass described in connection with this invention, particularly where the sliding connections are relatively secure or where the electrical variations to be measured are comparatively large and therefore not liable to be dwarfed by the eflect of a poor connection. Similarly, variations in the amplilying and recording systems may be made, and appropriate modifications therein will be obvious to those skilled in the art. Even in connection with the use of this invention in an impact test apparatus certain changes and improvements may be made without departing from the scope of this invention.

Accordingly it is to be understood that the scope and nature of the invention is not to be limited to the apparatus and equipment indicated in the specification but is to be construed in terms of the appended claims.

The invention described herein may be manuiactured and used by or for the Government of the United States of America for governmental purposes without the payment oi any royalties thereon or therefor.

What is claimed is:

1. In apparatus for testing material under tension. one or more tracks comprising electrical conductors disposed parallel to the direction of the test tension, a bar attached to the test specimen end to end in the direction or test tension. the bar having a large enough cross-section to v remain within its elastic limit under strain 0! the test tension, a strain gage of the electrical resistance type attached to the bar, an arm for each track attached to the bar and extending laterally therefrom towards the track, a shoe of conductor material at the end 0! each arm in position to engage and ride along the track. and an electrical conductor connecting each shoe with the strain gage.

2. In apparatus for testing material under tension, one or more tracks comprising electrical conductors disposed parallel to the direction of the test tension. a bar attached to the test specimen and to end in the direction of test tension. the bar having a large enough cross-section to remain within its elastic limit under strain of the test tension, a strain gage or the electrical resistance type attached to the bar, an arm for 1| 8. Inapparatus fortestingmaterialunder tension.oneormoretracksc0mprlsinge1ectrical conductors disposed parallel tothedirectionot thetest tensiomabarattachedtothetestspeelmenendtoendinthedirectionoi'testtenslon. the bar havingalargeenoughcross-secticnto remain within itselasticlimitunderstrainot the test tension, a strain gage resistance type attached tothe bar, an arm for each track attached to the bar laterallytherei'rom position to engage ride along the track, electrical conductor each shoe with the strain 8 8 and a potentiometer circuit inin circuit with to record variations or the resistance of the strain 8 8 tostrikeananviLoneormoretrackscomprising electrical conductors disposed parallel to the path of travel of the specimen assembly, a bar attached to the specimen y between one oi the masses and the incident end of the specimen. the bar having a large enough cross-section to remain within its elastic limit under strain oi thetesttensicmastraingageoitheelectrical resistancetypeattachedtothebananarmior eachtrackattachedtothebarandextending laterally therefrom towards the track. a shoe oi conductor material at the end or each arm in position to engage and ride along the track, and an electrical conductor connecting each shoe with the main gage.

5. In apparatus for testing material under impact tension by impelling a specimen assembly comprising a predetermined mass attached to each of opposite ends 01' a specimen of the materialtostrikeananviLabar-attachedtothe specimen between one oi the masses and the incident end of the specimen, the bar having a large enough cram-section to remain within itselasticlimitunderstrainotthetest tensiomastrainslseoitheelectricalresistanco typeattachedtothebanoneormoretraclrscomprising electrical conductors disposed on one side ofthepathoitravelofthe and disposed parallel thereto for the extent of its travel. a set or contacts for each track on the oppcnte side at the path of travel, each set oi contacts comprising opposed lengths of conductors disposed paralleltothepathoi'iu'avelofthe specimen assembly for the extent of its travel iromthetimeotimpactuntllthetestiscomplete.anarmioreachtrackattachedtothebar and extending laterally therefrom towards the trackinonedlrcctionandintheotherdirecticn towards the set 01' contacts, a shoe oi conductor materialattheendoieacharminpositlontoengageandridealongthetrack,aterminalpinat theotherendoieachsrmpositicnedtctravel between the opposed lengths of conductors in contact therewith, and electrical conductors conmeeting the shoes and terminal pins with the file or this patent:

UNITED STATES PA'IENTB strain gage. Number Name 7 Date WILLIAM H. HOPPMAN'N, II. 6 Lake Sept. 13, 1932 EDWARD C. TAYLOR. 5 ,985, 7 Yuasa. DEC. 25, 1934 2,178,422 Heagney Oct. 31, 1939 REFERENCES CITED 2,188,303 Roberts Jan. 30, 1940 The following references are of record In the 23625 simmms' 141 1944 

